1. Field of the Invention
This invention relates to method and apparatus for the real-time monitoring of welds using stress-wave emission techniques, and more particularly, to method and apparatus for monitoring welds by measuring the stress waves emitted during n time intervals of the weld cycle, where n.gtoreq. 4, and comparing the measurements obtained for the intervals with predetermined acceptable ranges for measurements selected from corresponding ones of the intervals and corresponding predetermined ratios between measurements for two or more intervals.
2. Description of the Prior Art
The ability to evaluate a weld using real-time, non-destructive methods has always been of interest to industry. A method for monitoring a welding operation is disclosed in U.S. Pat. No. 3,726,130, issued to R. P. Hurlebaus on Apr. 10, 1973. There, ultrasonic shear wave pulse signals are transmitted into the two pieces to be welded from a transducer positioned opposite the welding electrode while the welding operation is being performed. These signals are reflected from the area between the melting metal and the solid metal to provide real-time data for detecting the degree of penetration of a weld.
Another method for monitoring a welding operation is disclosed in an article entitled, "Forecasting Failures with Acoustic Emissions," by R. E. Herzog published in Machine Design, June 14, 1973, at pages 132-137. There it is stated that one of the more successful uses of acoustic emissions is in inspecting welds as they are being made by detecting and correlating signals emitted during the liquid-to-solid phase transformation of a weld area to indicate good or bad welds. The Herzog article further specifies that complex stress waves occur in both the weld cycle and post-weld cooling period, but only emissions during the post-weld cooling period are used for finding defects, such as cracks, as they occur in the weld area, and that emissions during the weld cycle are ignored.
It is also known to detect and measure the stress waves emitted from a weld area during a first solid-to-liquid phase transformation interval and a second liquid-to-solid phase transformation interval and then subtract the stress-wave energy measured during the second transformation interval from the stress-wave energy measured during the first transformation interval to provide an indication of the strength of the weld.
In any welding process, the region of two or more materials in intimate contact are melted and fused. The energy required for melting can be provided either by a current pulse as in resistance or capacitor discharge welding or by a radiation pulse from a laser. For an on-line determination of the quality and the extent of a weld, it is desirable to monitor the real-time evolution of the complete welding process; such as initiation of heating, solid-to-liquid phase transformation, fusion, and resolidification of the weld nugget, since each of these aspects, and others, can affect the quality and/or the extent of a weld. The problem still remains of providing method and apparatus which will evaluate the complete welding process.